Frequency Shift Keying (FSK) is one of the most frequently used digital modulation schemes. It finds widespread use in low to medium data rate applications. Demodulation is a process that takes place on the receive side of a communications link wherein the receiver recovers the baseband data bits from the received signal. The communications link can be wired or wireless link. In the receiver, the functional block that performs the demodulation process is called the demodulator. In an RF wireless receiver, an RF front-end processes the received signal to prepare it for demodulation before presenting it to the demodulator.
Digital FSK demodulators are attractive as they offer many advantages over their analog counterparts. Among these advantages are flexibility, possible early verification in a hardware setup, i.e., Field Programmable Gate Array (FPGA), scales up or down with technology with much less efforts than analog demodulators, easier integration with digital baseband processors, and their performance is much less affected by temperature, supply and process variations than the analog demodulators.
Digital FSK demodulators are widely used. Primarily they fall into one of two categories. One is based on using correlators, and the other one is based on using frequency discriminators. In binary FSK, a correlator demodulator uses two correlators, where one correlator performs a correlation function between an input signal (SIF) and a locally generated waveform that represents bit “1” (which is at frequency f1). The other correlator also correlates the signal SIF with a locally generated waveform that represents bit “0” (which is at frequency f0). The correlation operation is performed over one data bit duration. The output of the two correlators is then compared and a decision is then made whether the bit is “1” or “0”. In the frequency discriminator FSK demodulators, the frequency of the signal SIF is estimated by a discrimination circuit, which is simply a time-domain differentiator. Estimating what the signal SIF frequency is, determines whether the frequency is f1 or f0, and accordingly whether the data bit is “1” or “0”.
Although conceptually simple, the above digital FSK demodulators have complex implementations. This is because computing either the correlation or the time-domain derivative requires processing power that is not available on ultra-low power devices. Even if such processing power is available, the power consumption is normally high.
Therefore, there is a need for an all-digital FSK demodulator that is simple and does not require high computational power.
U.S. Pat. No. 7,026,864 B2 (Shiung) discloses an all-digital FSK demodulator. It performs two major functions. First, by oversampling an input signal (SIF), it detects the transitions and produces a “1” if a transition exists and a “0” if no transition is found. It then counts the number of ones and zeros. The number of ones is delivered to the Threshold Device which is then compared to the threshold level. Second, the Threshold Device uses a single threshold to determine whether the bit is “1” or “0”. While not necessary, it preferably sets the threshold to (f1+f0)/R, where f1 is the frequency that represents bit “1, f0 is the frequency that represents bit “0” and R is the data date.
While simple, there are two issues with Shiung. First, using a single threshold for bit decision makes the decision process subject to instantaneous errors due to system noise. This becomes more sensitive in a wireless environment due to interference from other systems. If the noise or interference level is such that the number of ones count is offset by one, a bit error results. The other issue is related to the appropriateness of this scheme in cases where the FSK modulation uses intermediate frequency steps as the baseband data bit changes. No provision is set by Shiung to differentiate between transitions due to intermediate frequency steps. Shiung is suitable for demodulating FSK modulated signals wherein two distinct frequencies are used only, one representing bit “1” and the other representing bit “0”.
Intermediate frequency steps in FSK modulation is of significant importance in wireless communications as it allows to spectrally shape the modulated FSK signal. Otherwise, the resulting FSK modulated signal may occupy a wider bandwidth, an issue known as spectral re-growth. In many cases, such bandwidth cannot be fit within the allocated RF spectrum (due to regulatory requirements), and either the data rate needs to be reduced or the signal be filtered by a bandpass filter at the RF carrier frequency. Neither of these options is desirable.